The invention relates to an electric motor with a commutator, preferably a universal electric motor, which is operated in series and ventilated by air flow, with a high power density, especially as a driving mechanism for electric hand-held tool devices which experience a high dust load, such as grinding implements, hammer drills, and the like.
Such electric motors consist of a rotor, which rotates about an axis of rotation and is constructed as an armature, with an armature winding, through which current flows, and a commutator, which switches over the terminal bases of the armature windings during the rotation of the rotor and includes the rotor and carbon brushes of the stator.
In the case of universal electric motors which, because of their high power density, are ventilated by air flow to dissipate the excitation heat, dust particles, carried along in the flow of cooling air, on the one hand, because of the high relative movement, leads to a roughening of the surface of the lamellas due to wear by blast air and, on the other, due to direct penetration between the lamellas and the carbon brushes, increases abrasive wear of the carbon brushes, which generally limits the lifetime of the commutator and, with that, of the electric motor or of the electric hand-held tool device.
According to U.S. Pat. No. 6,025,662, an electric hand-held tool device with an electric motor, ventilated by a flow of air, has a commutator, with a protective shield having two protective coverings, which are assigned to the carbon brushes and deflect the cooling air stream directly in front of the carbon brushes. The remaining lamellas continue to be exposed to the dust particles, which are carried along in the flow of cooling air.
The U.S. Pat. No. 4,311,935 describes a protective shield of a nonconductive material, which encloses an acute-angled sector of the commutator and is disposed, in the direction of rotation, ahead of the carbon brushes. The remaining lamellas continue to be exposed to the dust particles, which are carried along in the flow of cooling air.
According to U.S. Pat. No. 3,919,574, the commutator is encapsulated completely by a protective shield of a non-conducting material, such as a plastic. The protective shield peripherally surrounds the carbon brushes and, over a narrow, radial air gap, the lamellas. It is a disadvantage that the cooling of the commutator is reduced significantly because the protective shield consists of a plastic.
It is an object of the invention to provide a shield, which offers complete protection for the commutator of a universal electric motor, ventilated by air flow, against dust particles, which are carried along in the stream of cooling air, and enables the commutator to be adequately cooled.
Essentially, in the case of a universal electric motor, ventilated by air flow, the commutator is enclosed by a peripherally closed protective shield of a stiff and thermally conductive material, which surrounds with its internal casing radius at least the axial contact region, which is in frictional contact with the carbon brushes, and forms an insulating, radial air gap.
Due to the rotation of the lamellas about the axis of the rotor, a rotating air stream results within the annular air gap and brings about good thermal convection within the air gap as well as slight resistance to transfer of heat between the lamellas and air or air and the protective shield.
The stiff protective shield preferably consists of a thin sheet of metal, such as aluminum, and therefore has good thermal conductivity.
The surface of the protective shield advantageously is microscopically smooth on both sides. As a result, the resistance to the transfer of heat to the air is slight and there is little tendency for the dust particles to adhere.
Furthermore, the surface of the protective shield advantageously is dull and blackened, as a result of which good heat radiation is ensured.
Advantageously, the metal sheet has a profiled and preferably is folded in undulating fashion, as a result of which a high bending stiffness, as well as a larger surface area in relation to the internal radius of the casing, results.
The surface of the undulating, folded metal sheet advantageously proceeds helically about the axis of the rotor, as a result of which, due to the rotation of the lamellas, an air stream is formed along the individual surface perpendicularly to the lamellas. Over the axial fold openings, present on either side, this air stream enables air to be exchanged within the fold openings.
Advantageously, the lamellas additionally form an axially adjoining cooling region, which is not in frictional contact with the carbon brushes and within which the lamellas furthermore advantageously have small radial wing stubs, which contribute a better air circulation and add partially to the cooling of the lamellas.